1. Field of the Invention
This invention pertains to an in situ sensor for determining the degree and progress of corrosion of metal or alloy, for example steel, in a matrix structure. Methods of monitoring the progress of corrosion in such structures, as well as methods of determining the effectiveness of corrosion inhibitors and the effects on corrosion inhibitors of environmental dilution, are also disclosed.
2. Background of the Prior Art
The provision of a non-conductive matrix, defining a composition of matter, a structure, or a useful article, provided with conductive metal or alloy therein, is a common engineering composition. A wide variety of matrices are familiar to those of ordinary skill in the art. Among the most common matrices are concrete, refractory ceramics, polymeric coatings or masses (including reinforced resins, filled resins and foams) gels and vitrified glass like compositions. These compositions, and the articles and structures made therefrom, can all be described as being provided of a continuous matrix of the identified matrix material, with conductive elements incorporated in, typically embedded in, the matrix material. Thus, in concrete, steel reinforcement (rebar) is typically employed. Structures prepared from a polymeric matrix include high stiffness, high technology compositions such as aircraft and aerospace parts, which typically use reinforcing fibrous material such as carbon and graphite fibers, or fiberglass yarns, as well as lower strength filled resins, foams and the like. These may be provided with conductive elements for the purposes of reinforcement, to effect resistance heating, or to provide conductivity to the structure as a whole. Similarly, vitrified glass compositions may be provided with electrically conductive elements embedded therein, for example, to carry a current for the purpose of local heating, as in a windshield defroster, or as a reception antenna, for a radio and the like. Additionally, the matrix need not be solid. A wide variety of gels are employed across which a current may be desirably supplied. The most familiar of these matrices provided with metal or metal alloys is reinforced concrete, and this invention is discussed, in exemplary fashion, in terms of reinforced concrete. It will be apparent, however, that corrosion of steel or metal alloys embedded in a matrix is common to all of these structures. It is also common to metal or metal alloy structures that are embedded or coated with plastic film, such as underground storage tanks and the like.
In both concrete and high strength polymer matrixes, metal and metal alloys which are subject to corrosion are provided for reinforcement. The corrosion impacts the integrity and strength performance of the structure in question, and may reduce it below critical values.
The impact of reinforcing steel (rebar) corrosion on the performance life of concrete is now well recognized. For example, more than half of the bridges in the United States are affected by corrosion and approximately 20% have been deemed structurally deficient.
Rebar corrosion is of concern to manufacturers and users of concrete, rebar, and admixtures. This costly form of damage affects both building and highway construction, but there is no clear, reliable way to determine the rate of material loss of rebar embedded in concrete. Measuring the open circuit potential, for example, is easy and inexpensive but unreliable, producing errors three orders of magnitude and higher. Measuring linear polarization is complex, but is encumbered due to the potential drop in the concrete; electrochemical impedance spectroscopy can overcome that problem, but the results are extremely difficult to interpret and expensive to acquire.
One of the most important methods of mitigating rebar corrosion is through the addition of corrosion inhibitors into the concrete mix. Although such admixtures have been used commercially for years, there are no standard tests or specifications for their performance. Tests that are in use have serious shortcomings and do not fully exploit sophisticated electrochemical techniques.
To illustrate, the ASTM G-109 method (Wiss, Janey, Elstner Associates, Inc.) measures galvanic current between the top and bottom rebar mat during wet/dry cycling. Although galvanic polarization is a critical aspect of rebar corrosion, this test does not provide information on the total corrosion rate; it ignores the steady state corrosion current of the top-most layer, which could be significant. Moreover, corrosion in the mid-section of the bar may go undetected and misleading currents can be created by the elctroplater's tape required on the ends of the rebar. This test also cannot establish consistent galvanic couple from sample to sample and lacks statistical data and minimal criteria for inhibitor performance.
The search for a standard test is complicated by the diversity of inhibitors, which may be oxidizing or non-oxidizing anodic, cathodic, oxygen or chloride scavenging, film forming, and so on. Although manufacturers will naturally cite the test that makes their product stand out as a top performer, users require a more balanced approach for a realistic evaluation of all types of corrosion inhibiting admixtures.
There are also various electrochemical testing methods available: open circuit potential (E.sub.oc), polarization resistance (R.sub.p) via linear polarization and R.sub.p via electrochemical impedance spectroscopy (EIS). These methods must be used carefully, which is not the case in most reported studies. For example, real time correction (iR) for linear polarization is typically ignored, which causes underestimation of the corrosion rate. When the Stern Geary Equation and the anodic and cathodic Tafel slopes (B.sub.a and b.sub.c) are used, the literature generally assumes some value, such as 26 or 46 mV, for all inhibitors. Finally, most tests wrongly assume that the corrosion current is uniform over the surface.
The testing methods themselves also have inherent advantages and disadvantages. Open circuit potential is easy and nondestructive, but can be ambiguous; it reveals the relative ratio of anodic to cathodic areas but provides no information on reaction kinetics. Linear polarization and EIS provide accurate reaction kinetics data but take several hours per R.sub.p determination and limit the size of the test matrix, since only 10 samples per day can be run. Additional time may be needed to resolve ambiguities in the distributed element response in EIS spectra or in the Tafel slope in linear polarization data.
Developing an accurate test to evaluate corrosion requires a thorough understanding of concrete material science, corrosion, electrochemical test methods, and inhibitor evaluation. Our sensor provides sensitive, unambiguous corrosion data at low cost, and can monitor changes in corrosion aggressiveness over time. This sensor is particularly suitable for monitoring the performance of corrosion inhibitors under various conditions and for gathering large data pools for statistical analysis and standardization.
Our probe is designed to be a standard which can be used by engineers, technicians and scientists to assess the efficacy of commercial corrosion inhibiting admixtures or matrix chemistries or diffusion rates of aggressive species or mitigating agents. Our test is simple and easy, many more samples can be evaluated for more accurate trend and statistical analysis.